R语言拟合模型时，遇到问题，求大神指教
nihe=arma(x,order=c(2,1))
错误于AA %*% t(X) : 需要数值/复数矩阵/矢量参数
此外: 警告信息：
In ar.ols(x, order.max = k, aic = FALSE, demean = FALSE, intercept = include.intercept) :
  model order:  4 singularities in the computation of the projection matrix results are only valid up to model order 3